1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly to a nitride semiconductor light emitting device and a method of manufacturing the same, which can provide an improved electrode structure for the nitride semiconductor light emitting device to have uniform current diffusion and an increased current diffusion efficiency, thereby providing an enhanced brightness and driving voltage characteristics, and which can prevent a bonding wire from blocking a light emitting region when manufacturing a package, thereby preventing reduction in brightness after manufacturing the package.
2. Description of the Related Art
Recently, a light emitting device (LED) display board developed as a new transmission media for images or information has been advanced to a level of displaying a moving image, such as various CF images, graphic images, video display, etc., starting from information of simple characters or figures in the early days of the LED display board. With regard to colors, not limited to an existing monochromatic coarse display or at most a limited range of colors, such as red or yellow-green LEDs, in the past, as a high brightness blue LED using a nitride semiconductor has emerged recently, it has become possible to exhibit a full color display using colors of red, yellow-green and blue. However, since the yellow-green LED has a lower brightness than that of the blue LED or the red LED and emits light having a wavelength of 565 nm or so which is not the wavelength of green required for the three primary colors, it is not possible to display the full range of natural colors. These problems are overcome using a high brightness pure green nitride semiconductor LED emitting a wavelength of 525 nm suitable for displaying the full range of natural colors.
In such a nitride semiconductor light emitting device, since there is no commercially available substrate which has an identical crystal structure to that of a nitride crystal and which is in lattice matching with the nitride crystal, the nitride semiconductor material is grown on a sapphire substrate as a dielectric substrate. Since the nitride semiconductor light emitting device has the semiconductor material grown on the dielectric substrate, such as the sapphire substrate, electrodes cannot be formed on the back side of the substrate, necessitating formation of both electrodes on a crystal growth side of the semiconductor layer. An electrode structure of a conventional nitride semiconductor light emitting device and a cross-sectional structure thereof are exemplified in FIGS. 1 to 3.
FIG. 1a is a plan view of the conventional nitride semiconductor light emitting device, in which the conventional nitride semiconductor light emitting device 1 has an approximately rectangular shape and is formed with an n-type electrode 1n at one edge of the nitride semiconductor light emitting device and with a p-type electrode 1p at the center of an upper surface of the nitride semiconductor light emitting device. FIG. 1b is a cross-sectional view of the nitride semiconductor light emitting device taken along line l1–l1′ of FIG. 1a. As shown in FIG. 1b, the conventional nitride semiconductor light emitting device comprises: a sapphire substrate 11 of an approximately rectangular shape; a buffer layer 12 for preventing lattice mismatching in order to grow a nitride semiconductor material on the sapphire substrate 11; an n-type nitride semiconductor layer 13 formed on the buffer layer 12; an n-type electrode 17n formed on the n-type nitride semiconductor layer 13; an activation layer 14 formed on the n-type nitride semiconductor layer 13 such that an edge of the n-type nitride semiconductor layer 13 can be exposed; a p-type nitride semiconductor layer 15 formed on the activation layer 14; a transparent electrode layer 16 formed on the p-type nitride semiconductor layer 15 for providing an ohmic contact; and a p-type electrode 17p formed on the transparent electrode layer 16.
In general, the entire surface of the light emitting device can define a light emitting region (p-n junction), which emits light generated in the activation layer. However, in the electrode structure of the conventional nitride semiconductor light emitting device shown in FIG. 1, the p-type and the n-type electrodes are narrowly formed, thereby providing a low electric current diffusivity, and the p-n junction substantially contributing to the light emitting is formed around regions through which the electric current indicated by arrows in the drawings passes, thereby making it difficult to provide a high brightness. Additionally, as indicated by the arrows in FIG. 1a, the distance of current diffusion varies, making it difficult to provide uniform current diffusion, thereby reducing the brightness.
Further, since the conventional nitride semiconductor light emitting device shown in FIG. 1 has the p-type electrode formed at the center of the upper surface of the nitride semiconductor light emitting device, when the package is formed using a bonding wire, the light emitting region is blocked by the bonding wire, as indicated by symbol A′ of FIG. 4, so that there is a problem that the brightness is further reduced after the package is manufactured.
In order to solve the problems of the conventional nitride semiconductor light emitting device as described above, an enhanced nitride semiconductor light emitting device as shown in FIGS. 2a, 2b and 3 is developed. The nitride semiconductor light emitting device shown in FIGS. 2a and 2b is formed with an n-type electrode 2n or 27n of a band shape around the periphery of an upper surface of the n-type nitride semiconductor layer 23 and with a p-type electrode 2p or 27p at the center of the p-type nitride semiconductor layer 25 (or the center of the transparent electrode layer 26). As indicated by the arrows in FIG. 2a, since the diffusion of the electric current from the p-type electrode 2p or 27p to the n-type electrode 2n or 27n generally occurs over the entire light emitting region (p-n junction) in a constant distance, there is provided an advantageous effect that the diffusion of the electric current can be uniformly carried out, thereby enhancing the brightness.
However, there still exists the problem that since the p-type electrode 2p and 27p is disposed at the center of the light emitting region, the bonding wire blocks the light emitting region when manufacturing the package as shown in FIG. 4, thereby reducing the brightness. Additionally, the method of manufacturing the nitride semiconductor light emitting device shown in FIGS. 2a and 2b should be accompanied by an etching process for exposing some portions of the periphery of the n-type semiconductor layer 23 in order to form the n-type electrode 2n or 27n thereon, and be formed with an n-type electrode having a narrow band shape on the exposed region around the periphery of the n-type semiconductor layer 23, thereby resulting in a complicated manufacturing process. Further, the complicated and difficult process as described above increases manufacturing costs for the nitride semiconductor light emitting device. Further, there is provided a problem of damaging the n-type electrode, during a scribing process for dividing a plurality of light emitting device formed on a wafer into separate chips.
Meanwhile, the nitride semiconductor light emitting device as shown in FIGS. 3a and 3b has an n-type electrode 3n or 37n formed on the n-type nitride semiconductor layer 33 such that like the conventional nitride semiconductor light emitting device shown in FIGS. 1a and 1b, some portions of an edge of the n-type nitride semiconductor layer 33 are exposed, and a p-type electrode 3p or 37p formed at the edge of the p-type nitride semiconductor layer 25 (or at the edge of the transparent electrode layer 36) opposite to the edge formed with the n-type electrode. In such a conventional nitride semiconductor light emitting device, there are provided advantageous effects that as indicated by the arrows in FIG. 3, the diffusion of the electric current can be generally carried out at the light emitting region, and that as shown in FIG. 5, the bonding wire does not block the light emitting region B after the package is produced.
However, a longer diagonal line between the edges formed with respective electrodes provides a longer distance through which the electric current passes, resulting in a non-uniform diffusion of the electric current, so that the difference between the brightness at the center of the light emitting region and that at the surrounding portions thereof increases. Particularly, since two electrodes are formed as points, there is provided a problem of reducing total current diffusivity.
Thus, there is a need to provide a nitride semiconductor light emitting device and a method of manufacturing the same, which is formed with the p-type electrode and the n-type electrode such that uniform light emission can occur at the entire light emitting region having the p-n junction, thereby realizing high brightness characteristics.